Viscosity grading can be carried out on original (as-supplied) asphalt binder samples, referred to as AC grading, or on aged residue samples, known as AR grading. The AR viscosity test measures the viscosity of aged residue obtained from the rolling thin film oven test. AC grading characterizes asphalt binder properties before it undergoes the hot mix asphalt (HMA) manufacturing process. In contrast, the AR grading system aims to replicate asphalt binder properties after it has undergone a typical HMA manufacturing process, making it more representative of how asphalt binder behaves in HMA pavements. Table 1 provides an overview of the key advantages and disadvantages of the viscosity grading system.
Advantages and Disadvantages of Viscosity Grading:
Advantages:
- Represents Post-Manufacturing Behavior: AR grading better represents how asphalt binder behaves in HMA pavements, as it simulates post-manufacturing conditions.
- Realistic Performance Assessment: It provides a more realistic assessment of binder performance in the actual pavement environment.
- Better Aging Simulation: AR grading accounts for the aging effects that occur during the HMA manufacturing process.
- Improved Pavement Durability: Using AR grading can lead to better pavement durability predictions.
Disadvantages:
- Complex Testing: AR grading involves more complex testing procedures compared to AC grading.
- Limited Historical Data: There may be limited historical data available for AR-graded binders.
- Variability: Variability in the manufacturing process can affect the accuracy of AR grading results.
- Higher Costs: AR grading may involve higher testing costs compared to AC grading.
Please note that the advantages and disadvantages mentioned here are for informational purposes and may vary depending on specific applications and circumstances.
| Advantages | Disadvantages |
|---|---|
| Unlike penetration depth, viscosity serves as a fundamental engineering parameter | At 25°C (77°F), the main grading may not accurately represent the rheological behavior of asphalt binders at low temperatures |
| Test temperatures are in alignment with key pavement temperature benchmarks: 25°C (77°F) for average pavement conditions, 60°C (140°F) for high pavement temperatures, and 135°C (275°F) for HMA mixing temperatures. | In the AC grading system, the viscosities of thin film oven test residues within the same AC grade can exhibit substantial variations, leading to divergent post-construction performance among asphalt binders of the same grade. |
| The impact of temperature on asphalt binder rheology, known as temperature susceptibility, can be partially assessed through viscosity measurements carried out at three different temperature levels. (It’s worth mentioning that penetration measurements are exclusively conducted at 25°C or 77°F.) | Viscosity testing is not only a lengthier process but also incurs higher costs in comparison to the penetration test. |
| Testing equipment and standards are easily accessible and widely available. |
The new product grading method is now based on bitumen viscosity at 60°C and 135°C. This has led to the development of new grades with the following nomenclature:
| Standard | Grading Using Original Asphalt (AC): | Grading Using Aged Residue (AR): | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|
| AASHTO M 226 | AC-2.5 | AC-5 | AC-10 | AC-20 | AC-30 | AC-40 | AR-10 | AR-20 | AR-40 | AR-80 | AR-160 |
| ASTM D 3381 | AC-2.5 | AC-5 | AC-10 | AC-20 | AC-30 | AC-40 | AR-1000 | AR-2000 | AR-4000 | AR-8000 | AR-16000 |
| Standard | Minimum Absolute Grades | Viscosity at 600°C, Poise | Approximate Penetration Grade |
|---|---|---|---|
| IS73:2013 | VG 10 | 800 | 80-100 |
| VG 20 | 1600 | – | |
| VG 30 | 2400 | 60-70 | |
| VG 40 | 3200 | 30-40/40-50 |
Viscosity Grade vs. Penetration Grade: Addressing Key Issues
The transition from penetration grade to viscosity grade bitumen specifications has tackled several significant issues:
- Performance at High Temperatures: The adoption of viscosity-graded bitumen, determined by viscosity at 60°C rather than penetration at 25°C, addresses performance concerns at elevated temperatures. This shift is crucial for ensuring the durability of asphalt in hot climates.
- Compaction Challenges: Compaction issues caused by tender asphalt mixtures under roller wheels have been mitigated by introducing a minimum viscosity requirement at 135°C. This adjustment aims to minimize problems encountered during field applications.
- Reduced Testing Burden: The adoption of viscosity-graded paving bitumen specifications streamlines the testing process, reducing the total number of required tests to seven. This streamlining enhances efficiency without compromising bitumen quality or necessitating additional tests.
Viscosity-graded bitumen is classified based on its degree of fluidity, with higher grades indicating greater stiffness. These grades are determined through viscosity tests conducted at 60°C and 135°C, representing road surface and mixing temperatures during summer, respectively. Additionally, the penetration test at 25°C, reflecting the annual average pavement temperature, remains a part of the specifications.
By transitioning to viscosity grading, these specifications offer a more robust framework for assessing bitumen quality and performance in diverse environmental conditions.